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What is a rollback operation, what is it used for?
See What is a 'rollback'? and the bottom paragraph of the editing section in the help center.
Simply put, a rollback reverts an edit (or edits) on a post, returning it to a former state.
To roll a post back:
Click the timestamp below the post to view the edit history
Click the "rollback" button next to the revision you wish to revert to.